Monotony for seamen ordered to be confined on board (8)
I believe the answer is:
sameness
'monotony' is the definition.
('sameness' can be a synonym of 'monotony')
'seamen ordered to be confined on board' is the wordplay.
'ordered' indicates an anagram.
'to be confined' means one lot of letters goes inside another.
'on board' becomes 'ss' (I've seen this in other clues).
'seamen' anagrammed gives 'amenes'.
'amenes' going inside 'ss' is 'SAMENESS'.
'for' acts as a link.
(Other definitions for sameness that I've seen before include "Unchanging quality" , "Lack of variety" , "Tedious monotony" .)